A series of steps of a semiconductor device fabricating process includes a step of removing a resist coated on a surface of a semiconductor wafer. A known resist removing method makes a resist water-soluble and washes it out with deionized water. A process for making the resist water-soluble uses a substrate processing apparatus, which holds the wafer in a processing chamber, and supplies a mixed process fluid of ozone gas and steam into the processing chamber to make the resist into water-soluble by oxidizing the resist with the mixed process fluid. The substrate processing apparatus is provided with a steam generator that heats deionized water stored in a tank to generate steam to be supplied into the processing chamber. Steam generated by heating the deionized water in the tank flows upward, is carried by a steam supply line connected to an upper part of the tank, is mixed with ozone gas, and is supplied into the processing chamber.
A problem in the conventional substrate processing apparatus is that a metal forming the walls of the tank of the steam generator dissolves in the deionized water, and the dissolved metal flowed together with steam into the processing chamber forms particles, and the particles contaminate the wafer.
It is possible that the mist of deionized water flowed together with steam into the processing chamber forms water marks on the wafer. Therefore, the tank of the conventional steam generator must contain deionized water such that the level of deionized water is kept below the level of the steam supply line to prevent the flow of the mist of deionized water into the steam supply line. Since a large space needs to be formed between the liquid level in the tank and the steam supply line, the dimensional reduction of the tank is limited.
A steam generator capable of solving the foregoing problem is disclosed in U.S. Pat. No. 5,063,609 to Lorimer (JP3-137401A is the Japanese equivalent). Lorimer's steam generator is provided with a tank formed of a material from which metals do not dissolve into deionized water, such as Teflon®, to avoid the contamination of a wafer with metals. To solve the problem resulting from the mist of deionized water, Lorimer's steam generator has a plurality of stacked tanks each internally provided with a coil heater coated with Teflon. However, Lorimer's steam generator has a complicated construction, is costly and requires troublesome maintenance work. The coil heater has a narrow surface in contact with a liquid and hence its heating efficiency is low. It is possible that the Teflon coating is subject to deterioration with time.